A time-of-flight camera, also referred to as TOF camera, is a camera that usually comprises a light source or emitter unit for emitting modulated light pulses, a receiver unit that captures reflections of the light pulses, an evaluation unit, which evaluates time-of-flight information or the received reflections, and a calculation unit, which derives distance information from the time-of-flight information. The distance information is also referred to as depth. The emitter unit emits a modulated light pulse towards a scene, whereby the modulated light pulse is reflected from objects present in the scene towards the receiver unit. Depending on the distance of the objects from the TOF camera, the reflections are received with a delay in respect to the emitted modulated light pulse. This delay, also referred to as time-of-flight, is evaluated by the evaluation unit and further processed by the calculation unit to derive a distance of the objects.
The receiver unit comprises several light receiving points, also referred to as pixel, and an optical system, so that different pixels can receive reflections from different objects at the same time. Each of the pixels independently receives reflections of the modulated light pulses. Also the time-of-flight information and distance information are processed individually for each pixel, so that the distance information is provided for the entire scene simultaneously
The distance is measured pixel per pixel in an indirect manner by measuring the time delay or phase difference between sent and received modulated optical signal. Typically, the modulation can be a pulsed modulation, sinusoidal modulation, pseudo-noise modulation, etc. The phase difference of the sent and received modulation signal then offers a measure for the time delay.
To provide a TOF camera that can provide distance information with high accuracy, sharply defined pulses are used, which have preferably a limited rise and fall time. Due to this requirement the spectral content of the electronic modulation signal used to modulate the light output of electronic light sources contains a lot of harmonics with significant energy, as shown in the example spectrum of FIG. 1.
Also, the modulation frequency is usually well defined since the transformation from phase measurement to distance measurement requires accurate knowledge of the modulation frequency. Due to this, the peaks in the spectrum tend to be narrow but high.
Problems can arise, since TOF cameras must co-exist with other electronic devices. A lot of energy in the harmonics can prevent electromagnetic compliance of the device. Normally such a device has to comply with different norms. Relevant norms for electromagnetic compliance (EMC) are available according to the FCC (USA), EC (Europe) or CCC (China) standards. Accordingly, the use of TOF cameras can be restricted and/or the accuracy of TOF cameras can be limited indirectly by the requirement to fulfill EMC requirements. Typical modulation frequencies are between 10 MHz and 100 MHz, generating related harmonics.
Reducing the amount of EMI radiated from an electronic device is one of the hardest problems to resolve in the drive for lower production costs. Producing a compliant device can be quite costly if the necessary steps are not taken at design time. It is possible that 40-50% of the development cost of a new product can be spent in the quest for a compliant product suitable for economic production.
A clock oscillator generates a fixed frequency square wave signal used for timing in high-speed digital systems. The frequency of this clock is assumed to be fixed and taking the inverse of frequency gives the period of the clock. The period of the clock is the time from a point on the rising edge to the exact same point on the rising edge of the very next clock. An ideal clock would have no measurable jitter and the period of each clock cycle would always be exactly the same. A Low EMI clock oscillator or Spread Spectrum Clock is a special type of digital clock that provides lower EMI when compared with conventional clock generator outputs. The base frequency is modulated and the energy is spread out over a wider spectrum of frequencies, thereby reducing the peak energy contained at any one frequency. The peaks of the fundamental and harmonic frequencies are lower in relative strength. The total amount of energy that was originally in the harmonics of the base frequency clock signal does not simply disappear but rather is distributed over a wider band of frequencies. By varying the frequency of a clock, the period of such a clock is also changed which is the same as providing jitter. Thus cycle-to-cycle jitter is added to such a clock. Jitter will reduce the precision of range finding time-of flight devices in determining distance.